(1) Field of the invention
The present invention generally relates to a size magnification processing unit for processing a digital image in accordance with a magnification factor, and more particularly to a size magnification processing unit for processing a digital image in accordance with a magnification factor so that a digital image having a size corresponding to the magnification factor is obtained with little deterioration of quality thereof.
(2) Description of the related art
A digital image is obtained by sampling of image data (binary data or multi-value data) at original intervals on an original image. A size magnification process to be applied to a digital image corresponds to a resampling of image data on the digital image at virtual sampling points arranged at predetermined intervals different from the original intervals. For the sake of simplicity, FIG. 1 shows a size magnification process in which the sampling of image data is performed on a one-dimensional digital image. An original digital image is obtained by sampling of image data at original intervals (Io) as shown in FIG. 1(a). Image data is resampled on the original digital image at virtual sampling points arranged at intervals (I) corresponding to a magnification factor as shown in FIG. 1(c). Hereinafter, a magnification factor greater than 1 (100%) refers to enlargement and a magnification factor less than 1 (100%) refers to reduction. The resampled image data is arranged at the original intervals (Io) as shown in FIG. 1(d), so that a digital image having a size corresponding to the desired magnification factor is obtained. When the magnification factor is less than 1 (100%), the sampling intervals (I) corresponding to the magnification factor are greater than the original sampling intervals (Io), as shown in FIG. 1(a) and (c). In this case, a reduced digitalized image is obtained as shown in FIG. 1(d). When the magnification factor is greater than 1 (100%), the sampling intervals (I) corresponding to the magnification factor are less than the original sampling intervals (Io). In this case, an enlarged digital image is obtained.
The intervals (I) at which the image data is resampled on the original digital image differ from the original intervals (Io) used in the sampling to obtain the original digital image, as shown in FIG. 1(a) and (c). Thus, image data to be resampled at the intervals (I) on the original digital image is obtained using an interpolation technique as shown in FIG. 1(b). A linear interpolation method is well known as one type of interpolation technique. When a digital image is processed in accordance with this linear interpolation method, blurred areas occur at edges of an image obtained by the process.
The applicant has proposed, in Japanese Patent Application No.4-79976, a non-linear interpolation process applicable to the size magnification process. According to the non-linear interpolation process proposed by the applicant, the blur areas occurring at the edges of an image obtained by the process can be improved.
However, in a case where the original image includes various patterns other than line drawings, when the original image is processed in accordance with the size magnification process, not only the blur areas occur, but also moire and/or deformation of texture occur so that the quality of an image obtained using the size magnification process deteriorates. This quality deterioration (the moire and/or the deformation of texture) of the image may be based on aliasing noises generated by the resampling of the image data in the size magnification process.
A digital image obtained by scanning a dotted image on a printed paper includes various high-frequency components. In a pseudo half-tone image formed in accordance with a dither method or an error diffusion method, the half-tone is represented by pixel number ratio of black and white pixels on each local area. Thus, the pseudo half-tone image also includes a high-frequency component peculiar to each local area. When the above image is processed in accordance with the size magnification process, as shown in FIG. 1, a spectrum S1 generated by the sampling of the image data at the predetermined sampling frequency in the size magnification process overlaps with a high-frequency region of a space frequency spectrum So of the original digital image as shown in FIG. 2. Due to the overlapping of the spectrums S0 and S1, the aliasing noise occurs. That is, in a case where a systematic dither image having a strong periodicity is processed, the moire occurs, and in a case where a pseudo half-tone image obtained by the error diffusion method is processed, the disorder of the texture occurs.
As has been described above, when the original digital image is processed in accordance with the size magnification process, the quality of the image deteriorates based on the high-frequency components included in the original digital image. To improve the quality of the image obtained by the size magnification process the high-frequency components may be removed from the original digital image before the original digital image is processed. An image processing unit has been proposed in Japanese Laid Open Patent Application No.1-134577, in which unit a smoothing process of the original digital image is performed before the size magnification process.
However, a frequency region based on which the moire and the disorder of the texture occur depend on a magnification factor desired and on the type of original digital image. For example, the smaller the magnification factor, the larger the intervals at which the data is sampled on the original digital image and the lower the central frequency of the spectrum S1 generated by the sampling.
On the other hand., when a binary image (e.g. line drawings) is processed by the size magnification process, the moire and the disorder of the texture do not occur. To show clear edges, the high-frequency components of the original binary image is not to be removed therefrom.